In the past few years the development of safety systems for elevators has been directed towards the replacement of existing analogue safety circuits having series-connected safety contacts by bus-based electronic safety systems.
The patent EP 2022742 A1 shows an example of such a bus-based electronic safety system. That document relates to a decentralized safety system comprising two safety control units. One safety control unit is arranged on the car and a further safety control unit is assigned to the shaft. Both of the safety control units are connected via a safety-based bus. The one safety control unit on the car performs the task of monitoring all position-dependent and velocity-dependent safety-relevant motion states of the car. The other safety control unit, by contrast, primarily monitors safety contacts, such as shaft door contacts or shaft end contacts.
The decentralized safety system presented in EP2022742 A1 follows the premise of evaluating the locally available sensor and contact signals by means of the locally arranged safety control unit and monitoring the safety functions which are dependent thereon. Thus, for example, the one safety control unit evaluates the position and speed signals available on the car and compares them with a set of limit curves stored on the safety control unit. If the speed of the car exceeds a limit value specified for a certain position, then the first safety control unit triggers the drive brake or the safety brake. Accordingly, the other safety control unit monitors the condition of the shaft door contacts, for example, and after detecting an invalid safety condition of a shaft door contact, triggers the drive brake or the safety brake in turn. An invalid safety condition for a shaft door exists, for example, if the shaft door of a landing is open but at the same time no car is present on the corresponding floor.
A disadvantage of this safety system is that the computing power available on the one safety control unit for monitoring the position-dependent and velocity-dependent safety-relevant motion states of the car is relatively high in comparison to the computing power available on the other safety control unit for monitoring the safety contacts. Accordingly, the first safety control unit is relatively expensive to procure.